The present invention relates to a method and/or architecture for implementing a programmable impedance generally and, more particularly, to a method and/or architecture for implementing a common programmable impedance pin for monolithic devices that may be used by at least two dies that each have separate programmable impedance circuitry.
Conventional ZQ-technology provides programmable impedance for single dies. Conventional single-die ZQ-circuitry does not allow for easy expansion by multiple die techniques, such as die stacking. Connecting the ZQ pad of all the dies in a package to a single pin drives current into all the pins. The current causes deviation from the resistance value on a single die, which would typically be in the range of 5 times the required impedance. Moreover, the circuitry on each of the dies responds at different times causing the effective output impedance to change erroneously.
Referring to FIG. 1, a typical single-die ZQ-technology programmable impedance circuit 10 is shown. The circuit 10 illustrates a programmable impedance architecture for QDR(trademark) SRAMs (QDR(trademark) is a trademark of Cypress Semiconductor, Inc., San Jose, Calif.). The circuit 10 includes a circuit 12 and a circuit 14. The circuit 12 has an input 16 that receives the signal ZQPAD. The circuit 12 also has an output 18 that presents the signal ZQCLK, an output 20 that presents the signal ZQLOAD and an output 22 that presents the signal ZQDATA. The circuit 14 has an input 24 that receives the signal ZQCLK, an input 26 that receives the signal ZQLOAD and an input 28 that receives the signal ZQDATA. The circuit 14 also has an output 30 that presents the signal O/P_PAD. The circuit 10 provides programmable impedance for a single die.
A designer typically chooses a,resistive value to program the impedance of a die by selecting a resistor that is five times the desired impedance value. The resistor is connected between ground and the pad ZQPAD. The ZQ circuitry 12 sends out a current and measures the resulting voltage on the pad ZQPAD. Based on the result, the ZQ-circuit 12 sends three signals (ZQCLK, ZQLOAD, and ZQDATA) to, the programmable output drivers 14. Such a configuration may allow the designer to match the output impedance of the device to the transmission line impedance on a printed circuit board, resulting in improved signal integrity. The drivers 14 use the data in the ZQ-signals to set the impedance of the signal O/P PAD.
It is generally desirable to implement ZQ-technology to implement a common programmable impedance pin for multiple-die implementations within the same package.
The present invention concerns an apparatus comprising a first circuit and a second circuit. The first circuit may be configured to generate one or more first control signals in response to a reference impedance. The second circuit may be configured to operate in (i) a first mode in response to a first state of a second control signal and (ii) a second mode in response to a second state of the second control signal. When the second circuit is in the first mode, an output impedance of the second circuit may be adjusted in response to the one or more first control signals and the one or more first control signals may be presented at a first input/output of the second circuit. When the second circuit is in the second mode, the output impedance of the second circuit may be adjusted in response to one or more third control signals received at a second input/output of the second circuit.
The objects, features and advantages of the present invention include providing a method and/or architecture for implementing a common programmable impedance pin for monolithic devices that may be used by at least two dies that each have separate programmable impedance circuitry that may (i) allow many device dies to be stacked together in the same package, (ii) keep existing pin configurations constant, (iii) allow programmable output impedance for stacked dies, and/or (iv) allow ease of expansion.